The present invention relates to methods of materials processing that use pulsed lasers. In particular, it relates to methods for increasing the rate and precision of material removal by pulsed laser radiation.
Conventional mechanical lathes and machine tools (e.g., slitting saws) are effective for cutting/drilling materials down to approximately 100 microns kerf width at depths on the order of 1 millimeter (aspect ratio  less than 10:1). Below this level, electron beam or laser tools are typically used for cutting or high precision machining (sculpting, drilling). The majority of electron beam and existing industrial laser technology remove material by a localized thermal process where the material to be removed is heated to the melting or boiling point. Laser processing by molecular dissociation in organic (and some inorganic) materials can be achieved with ultraviolet lasers but this photodissociation mechanism is not applicable to all materials.
The basic interaction in localized thermal processing as is achieved with electron beam or current state of the art lasers is the deposition of energy from the incident beam in the material of interest in the form of heat (lattice vibrations). A continuous or pulsed laser beam is directed towards a set of optics, which focuses the beam onto the workpiece. In the case of pulsed lasers, the beam 100 consists of a train of individual pulses 102 with a duration typically between 10 and 500 nanoseconds, e.g., 100 nsec, and a repetition rate between 0.1 and 100 kilohertz (see FIG. 1). Absorption of beam energy may differ strongly between materials dependent upon the thermomechanical properties of the metal. Laser absorption is also dependent upon the optical properties of the material of interest. Metals absorb laser energy quite differently than dielectrics for example.
The laser energy that is absorbed results in a temperature increase at and near the absorption site. As the temperature increases to the melting or boiling point, material is removed by conventional melting or vaporization. Depending on the pulse duration of the laser, the temperature rise in the irradiated zone may be very fast resulting in thermal ablation and shock. The irradiated zone may be vaporized or simply ablate off due to the fact that the local thermal stress has become larger than the yield strength of the material (thermal shock). In all these cases, where material is removed via a thermal mechanism there is an impact on the material surrounding the site where material has been removed. The surrounding material will have experienced a large temperature excursion or shock often resulting in significant change to the material properties. These changes may include a change in grain structure, microfracturing or an actual change in composition. Such compositional changes include oxidation (if cut in air or, in the case of alloys, changes in composition of the alloy. This effected zone may range from a few microns to several millimeters depending on the thermomechanical properties of the material, laser pulse duration and other factors (e.g., active cooling). In many applications, the presence of the heat or shock effected zone may be severely limiting since the material properties of this zone may be quite different from that of the bulk.
Another limitation of conventional laser or electron beam processing in high precision applications is the presence of redeposited or resolidified material. As mentioned previously, cutting or drilling occurs by either melting or vaporizing the material of interest. The surface adjacent to the removed area will have experienced significant thermal loading often resulting in melting. This melting can be accompanied by flow prior to solidification and the deposition of slag surrounding the kerf. In many high precision applications, the presence of slag is unacceptable. Also, redeposition of vaporized material on the walls or upper surface of the kerf is common. This condensate often reduces the quality of the cut and decreases the cutting efficiency since the beam must again remove this condensate before interacting with the bulk material underneath.
Many of these limitations can be reduced by the use of secondary techniques to aid the cutting process. The most common of these are active cooling of the material of interest either during or immediately following the laser pulse, and the use of high pressure gas jets to remove vaporized or molten material from the vicinity of the cut to prevent redeposition. These techniques can be effective at improving the kerf at the cost of a significant increase in system complexity and often a decrease in cutting efficiency.
The use of lasers employing extremely short (less than 10xe2x88x9210 seconds) pulses has recently been introduced to machine materials with extremely high precision, such as described in U.S. Pat. No. 5,720,894 entitled ULTRASHORT PULSE, HIGH REPETITION RATE LASER SYSTEM FOR MATERIAL PROCESSING, issued Feb. 24, 1998, which is incorporated herein by reference. This technique utilizes a non-thermal mechanism to remove material, such as described in U.S. Pat. No. 6,150,630 entitled LASER MACHINING OF HIGH EXPLOSIVES, issued Nov. 21, 2000, which is incorporated herein by reference. While the mechanism has been shown to achieve extremely high precision with negligible collateral damage to the remaining material, the pulse processing rate (volume of material removed per laser pulse) and the processing efficiency (grams of material removed per Joule of laser energy) is limited.
In most industrial machining processes, once an acceptable level of precision and collateral damage is achieved, attention then shifts to optimizing the processing rate and efficiency. Increases in processing rate have been achieved by a number of techniques including adjusting the laser wavelength for specific materials, beam shaping, trepanning, gas assists, etc. Nearly all of these techniques are material dependent and lead to a significant increase in the complexity of the laser machining system.
The present invention advantageously addresses the needs above as well as other needs by providing a method and apparatus for material modification utilizing a burst of laser pulses where the pulse durations and timing between bursts are controlled to enhance material removal rate.
In one embodiment, the invention may be characterized as a method for material modification and a means for accomplishing the method, the method comprising the following steps: providing bursts of laser pulses, wherein each burst comprises at least two laser pulses, wherein each laser pulse has a pulse duration within a range of between approximately 10 ps and 100 ns, wherein a time between each laser pulse of each burst is within a range of between approximately 5 ns and 5 xcexcs; a time between successive bursts is greater than the time between each laser pulse comprising each burst; and directing the bursts upon a workpiece, wherein an intensity of a primary laser pulse of each burst exceeds a damage threshold of the workpiece.
In a further embodiment, the invention may be characterized as a method for material modification comprising the steps: providing bursts of laser pulses, wherein each burst comprises at least two laser pulses, a time between successive bursts is greater than a time between each laser pulse comprising each burst; directing the bursts upon a workpiece, wherein an intensity of a primary laser pulse of each burst exceeds a damage threshold of the workpiece; wherein a primary pulse of each burst produces an ablation plasma and an ejecta; and wherein a secondary pulse of each burst is timed to occur after substantial dissipation of the ablation plasma and to interact with the ejecta, whereby forming a heated material that interacts with the workpiece.
In another embodiment, the invention may be characterized as a method for material modification, comprising the steps: providing bursts of laser pulses, wherein each burst comprises at least two laser pulses, a time between successive bursts is greater than a time between each laser pulse comprising each burst; directing the bursts upon a workpiece, wherein an intensity of a primary laser pulse of each burst exceeds a damage threshold of the workpiece; wherein a primary pulse of each burst produces a first material phase to a temperature greater than 20,000 K and a second material phase to a temperature less than 10,000 K; and wherein a secondary pulse of each burst is timed to primarily interact with the second material phase.
In a further embodiment, the invention may be characterized as an apparatus for material modification comprising: a laser configured to produce laser pulses having a pulse duration within a range of between approximately 10 ps and 100 ns, a time in between the laser pulses greater than the pulse duration; a beam splitter configured to split each laser pulse into split laser pulses; a laser path traveled by each of the split laser pulses, each laser path having different lengths; and a beam combiner configured to receive each of the split laser pulses and direct the split laser pulses as a burst onto a workpiece. The different lengths of the each laser path are configured to cause a time of arrival at the workpiece between the split laser pulses to range between 5 ns and 5 xcexcs; and an intensity of a primary pulse of the split laser pulses comprising each burst exceeds a damage threshold of the workpiece.